Resin stuck on the mould wall

[drodrig]

Hi there,

We are building some CFRP boxes with a metal mould. We are using aluminium G.AL® C330 (EN AW 7021 [AlZn5,5Mg1,5] from Gleich Germany) and Prepreg SIGRAPREG® C W205-TW2/2-E323/43%

We cured 3 hours at 120ºC (248F).

The unmoulding wasn't easy. It got stack on the inner walls of the mould.

We used 3 layers of Loctite Frekote 770NC. In the past we used it on a stainles steel mould with a E022 resin without problems

Attached some pictures

What do you think the problem could be? Maybe it was cured too long?

thanks
regards,

 

[ctopher]

It's been over 6 years since I have done molds like this. But, when I did, for temps shown, we used steel not alum for this reason. Too hot for alum.

ctopher, CSWP
SolidWorks '17
ctophers home
SolidWorks Legion

 

[Compositepro]

Mold release problems are particularly common on new molds. New mold surfaces need to be thoroughly cleaned with solvent before applying the release and then five coats of release should be applied. The coating needs to be allowed to cure completely between coats. This may seem excessive and overly conservative but experience has shown that even with 5 coats, the first part release can be difficult. Watch closely as you apply coats at how the liquid release beads-up when initially applied. The first coat will not bead-up much, and you will notice improved beading as more coats are applied. This is time consuming, but not nearly as bad as a part that does not release. Rushing through this process is basic to human nature, and thus very difficult to prevent, except by having workers who have experienced the problems first hand.

770NC is a moisture curing material. It cures by reacting with moisture in the atmosphere. Room humidity and temperature are important factors in how long each coat takes to cure. Each coat must be buffed after it is cured and before another coat is applied. There should be no visible liquid beads left on the mold surface after each application.

Other factors to consider are:

>Liquid resins are reactive chemicals, and aluminum is a very reactive metal. They can be corrosive to aluminum . Most resins are also somewhat electrically conductive during cure. Carbon fiber in contact with aluminum forms a battery. Any electrical current flow will corrode the aluminum and generate hydrogen gas. Thus the release coating needs to be thick enough to be impervious to liquid resin and to prevent electrical contact between carbon and aluminum.

>Any air bubbles or other voids can result in resin residue left on the mold surface. Resin cured in contact with a mold surface will stick more strongly to the mold than to a void.

 

[drodrig]

ctopher,
What about 80°C (175F)? This mould has some mandrels (inner negatives) we used in the past of aluminium 7075 and it was never a problem. Maybe this new, shiny G.AL® C330 (7021) is worse regarding the resin getting stuck?

Compositepro,
We have cleaned the new parts with Frekote PMC, but honestly not so thoughtfully and then added 3 coats of 770NC. I didn't know it was so crucial. In the past (stainless steal mould, alu 7075) 3 layers was always fine.

The release agent was wiped with the open window in order to have better ventilation, so with the winter, we had no real room temperature.

I now understand the importance of the release agent.

What can we do now to clean it? I tried with the Frekote PMC but it is not really working. If I use a tool with a blade it will scratch it. Is there any chemical solution? or we have to go to something "more mechanical"?

thanks for the valuable information
regards,

 

[Compositepro]

A buffing compound with mild abrasive will remove release build-up and corrosion. For your sticking resin you could start with a fine sandpaper or steel wool.

One of the best release materials out there is non-porous, teflon-impregrated fiberglass. You can buy sheets as thin as 0.005" with a pressure sensitive adhesive, which can be applied in minutes to flat surfaces. I have never seen a release problem with this but it is not drapeable, so surfaces with compound curvature are challenging to cover. Molded surfaces will be very good quality, but you cannot get a mirror finish. There is always some print-through of the fiberglass fabric in the teflon sheet. However, the fabric gives the sheet dimensional stability, durability, and low thermal expansion.

 

[btrueblood]

"

https://tdsna.henkel.com/americas/n...8525715C001BD4D5/$File/FREKOTE 770-NC-EN.pdf"

Top right of the first page:
Fresh or "production line"
repairs, new fiberglass and epoxy molds should be cured per
manufacturer's instructions, usually a minimum of 2 -3 weeks
at 22°C before starting full-scale production.

Wow, 3 weeks?

 

[Compositepro]

The 3 week cure refers to cure of molds made from room temperature cure materials. Reaction rates are greatly accelerated with heat, but not all manufacturers have ovens for large molds or parts, hence the use of 2-part, room temperature curing resins.

 

[drodrig]

The teflon-impregnated fiberglass sheets look promising. We don't care about the mirror finishing and we have flat surfaces.

The problem is that we have high mechanical tolerances and small CFRP wall thickness (300 microns). The sheets would make our final part smaller, or even not possible (0.005" = 127 microns, our wall: 300)

If we cure at a lower temperature i.e. 80C (175F), clean the mould thoroughly (with Frekote PMC), apply 5 coats of release agent (Frekote 770NC) would it be better?

I don't understand why with the aluminium 7075 we had no problems and now with the G.AL® C330 (7021) we do

thanks,

 

[Compositepro]

"I don't understand why with the aluminium 7075 we had no problems and now with the G.AL® C330 (7021) we do."

That probably does not have anything to do with your problem. It is just a coincidental factor.

You can also run the teflon sheet onto clamping surfaces to act as a spacer that keeps the mold gap from changing. The problem you are experiencing can also be caused by excessive clamping pressure on the fabric when closing the mold. Trying to clamp materials to 300 micron thickness seems like a very questionable proceedure. Excessive contact pressure on the release coating can easily damage the coating and also crush the carbon fiber.

 

[drodrig]

Maybe because we cleaned it very well at the beginning? and now with the use it doesn't stick anymore?

Also I must say that the new one (G.AL® C330 (7021)) has better surface finishing. Maybe this also plays a roll

 

[btrueblood]

Thanks CP, I thought they were referring to molds "for" glass/epoxy, not "of" glass/epoxy. Makes sense now.